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Ch06 Presentation Transcript

  • 1. Chapter 6: AtmosphericMoistureMcKnight’s Physical Geography:A Landscape Appreciation,Tenth Edition, Hess
  • 2. © 2011 Pearson Education, Inc.Atmospheric Moisture• The Impact of Moisture on the Landscape• The Hydrologic Cycle• The Nature of Water: Commonplace butUnique• Phase Changes of Water• Water Vapor and Evaporation• Measures of Humidity• Condensation2
  • 3. © 2011 Pearson Education, Inc.Atmospheric Moisture• Adiabatic Processes• Clouds• The Buoyancy of Air• Precipitation• Atmospheric Lifting and Precipitation• Global Distribution of Precipitation• Acid Rain3
  • 4. © 2011 Pearson Education, Inc.The Impact of Moisture on theLandscape• Formation of fog, haze, clouds, and precipitation• Short term impacts of precipitation—floods• Longer term impacts (i.e., caves) on Earth’ssurface4
  • 5. © 2011 Pearson Education, Inc.The Hydrologic Cycle5Figure 6-1
  • 6. © 2011 Pearson Education, Inc.The Nature of Water:Commonplace but Unique• Chemistry of water– Atoms and molecules– Two hydrogen and oneoxygen molecule (H2O)– Covalent bonds– Electrical polarity ofwater molecule– Hydrogen bonds6Figure 6-2
  • 7. © 2011 Pearson Education, Inc.The Nature of Water:Commonplace but Unique• Important properties of water– Exists as a liquid at most points on Earth’s surface– Expands when it freezes; less dense than liquidwater; ice floats in water– Hydrogen bonding creates surface tension, a “skin” ofmolecules giving water a stickiness quality– Capillarity– Good solvent– High specific heat7
  • 8. © 2011 Pearson Education, Inc.Phase Changes of Water• Water typically exists inthree states– Solid: ice– Liquid: liquid water– Gas: water vapor• Latent heat is requiredto convert water to itsdifferent phases8Figure 6-4
  • 9. © 2011 Pearson Education, Inc.Phase Changes of Water• Phase change processes– Condensation: gas toliquid– Evaporation: liquid to gas– Freezing: liquid to solid– Melting: solid to liquid– Sublimation: solid to gasand gas to solid• Latent heat required foreach process• Latent heat as a sourceof atmospheric energy9Figure 6-5
  • 10. © 2011 Pearson Education, Inc.Water Vapor and Evaporation• Properties of water vapor– Colorless, odorless,invisible– Air feels sticky• Evaporation– Warmer temperaturesevaporate more water– Vapor pressure– Windiness reducesevaporation– Evapotranspiration10Figure 6-6
  • 11. © 2011 Pearson Education, Inc.Measures of Humidity• Humidity—amount of watervapor in the air• Absolute humidity—mass ofvapor for a given volume ofair• Specific humidity—mass ofwater vapor for a given massof air• Vapor pressure—contributionof water vapor to totalatmospheric pressure11Figure 6-7
  • 12. © 2011 Pearson Education, Inc.Measures of Humidity• Relative humidity—how closethe air is to saturation• Saturation represents themaximum amount of watervapor the air can hold• Saturation depends ontemperature• Saturation vapor pressure12Figure 6-8
  • 13. © 2011 Pearson Education, Inc.Measures of Humidity• Relative humidity—examplecalculation– Assume air at 20°C has 10 g ofwater vapor per kg of dry air– To calculate relative humidity,use the curve to get saturationconditions at 20°C (15 g/kg)– RH = (10g/15g) X 100% = 66.7%13Figure 6-8Saturation specific humidity attemperature of 20°C
  • 14. © 2011 Pearson Education, Inc.Measures of Humidity• Temperature and relativehumidity are inversely related• Dewpoint temperature• Sensible temperature14Figure 6-9
  • 15. © 2011 Pearson Education, Inc.Condensation• Conversion of vapor to liquid water• Surface tension makes it nearlyimpossible to grow pure waterdroplets• Supersaturated air• Need particle to grow dropletaround, a cloud condensationnuclei• Liquid water can persist attemperatures colder than 0°Cwithout a nuclei—supercooled15Figure 6-10
  • 16. © 2011 Pearson Education, Inc.Adiabatic Processes• Definition of adiabaticprocess• Dry adiabatic lapse rate• Lifting condensation level(LCL)• Saturated adiabatic lapserate• Parcel lapse rates versusenvironmental lapse rate16Figure 6-13
  • 17. © 2011 Pearson Education, Inc.Clouds• Definition of clouds• Influence on radiantenergy• Classification (3 primarycloud forms)– Cirrus clouds17Figure 6-15a
  • 18. © 2011 Pearson Education, Inc.Clouds– Stratus clouds– Cumulus clouds18Figure 6-15bFigure 6-15c
  • 19. © 2011 Pearson Education, Inc.Clouds• Cloud types– High clouds (over 6 km)– Middle clouds (from 2 to6 km)– Low clouds (less than 2km)– Clouds of verticaldevelopment• Grow upward from lowbases to heights of over15 km occasionally19Figure 6-16
  • 20. © 2011 Pearson Education, Inc.Fog20Figure 6-18
  • 21. © 2011 Pearson Education, Inc.Dew and Frost• Dew– Usually originates from terrestrialradiation– Moisture condensation on surfacesthat have been cooled to saturation– Will appear as water droplets• Frost– Simply a cloud on the ground– Occurs when air temperature lowersto saturation point, when thesaturation point is below 0°C (32°F)– Will appear as large numbers ofsmall white crystals21Figure 6-20
  • 22. © 2011 Pearson Education, Inc.The Buoyancy of Air• Definition of buoyancy• Stable air—parcel isnegatively buoyant, willnot rise without anexternal force• Unstable air—parcel ispositively buoyant, will risewithout an external force• Conditional instability22Figure 6-21
  • 23. © 2011 Pearson Education, Inc.The Buoyancy of Air• Determination ofstability via temperatureand lapse rate• Stable• Unstable23Figure 6-23Figure 6-24
  • 24. © 2011 Pearson Education, Inc.The Buoyancy of Air• Conditional instability• Visual determination ofinstability24Figure 6-26Figure 6-25
  • 25. © 2011 Pearson Education, Inc.Precipitation• Originates from clouds• Condensation insufficient toform raindrops• Other processes important• Collision/coalescence—tinycloud drops collide andmerge to form larger drops25Figure 6-27
  • 26. © 2011 Pearson Education, Inc.Precipitation• Ice crystal formation– Bergeron process– Ice crystals and supercooleddroplets coexist in coldclouds– Ice crystals attract vapor,supercooled drops evaporateto replenish the vapor– Ice crystals fall as snow orrain26Figure 6-28
  • 27. © 2011 Pearson Education, Inc.Precipitation• Types of precipitation– Rain: liquid water– Snow: cloud ice crystals– Sleet: snow melted andfrozen again before hittingland, ice pellets– Glaze (Freezing Rain): waterfalls as liquid, freezes tosurfaces– Hail: strong updrafts arerequired27Figure 6-30
  • 28. © 2011 Pearson Education, Inc.Atmospheric Lifting• Four types of atmospheric lifting28Figure 6-32
  • 29. © 2011 Pearson Education, Inc.Global Distribution ofPrecipitation• High precipitation regions, tropics• Low precipitation regions, deserts and poles29Figure 6-34
  • 30. © 2011 Pearson Education, Inc.Global Distribution ofPrecipitation30Figure 6-35
  • 31. © 2011 Pearson Education, Inc.Global Distribution ofPrecipitation31Figure 6-37
  • 32. © 2011 Pearson Education, Inc.Acid Rain• Definition of acid rain• Sources of acid rain• Principal acids—sulfuric and nitric• Number of hydrogenions—pH32Figure 6-38
  • 33. © 2011 Pearson Education, Inc.Acid Rain• Distribution of acid rain in the United States33Figure 6-39
  • 34. © 2011 Pearson Education, Inc.Summary• Moisture can impact the landscape in a variety ofways, including fog, haze, and precipitation• The hydrologic cycle shows the balance betweenwater removed from the oceans and water returnedby precipitation• Water has a number of unique properties• Water vapor is the gas form of water• Evaporation rates change as surroundingatmospheric conditions change• There are several measures of vapor content in theatmosphere 34
  • 35. © 2011 Pearson Education, Inc.Summary• There are several measures of vapor content in theatmosphere, called humidity measurements• Condensation is the process by which vapor isconverted to liquid• Adiabatic processes explain changes in parceltemperature without the addition or subtraction ofheat to the parcel• Clouds are a visual identification of saturation• Air has buoyancy associated with it that describesits stability35
  • 36. © 2011 Pearson Education, Inc.Summary• Many processes are responsible for precipitation• There are five primary types of precipitation• Atmospheric lifting occurs through four primarymechanisms• The most highly variable rainfall worldwide occursover deserts• Tropical regions are generally wet• Acid rain affects the Northeast and results fromcompounds released into the air by humans36